CA2836760C - Co-culture lymphoid tissue equivalent (lte) for an artificial immune system (ais) - Google Patents

Abstract

The present invention relates to methods for preparing an artificial immune system. The artificial immune system comprises a cell culture comprising T cells, B cells and antigen-primed dendritic cells. The artificial immune system of the present invention can be used for in vitro testing of vaccines, adjuvants, immunotherapy candidates, cosmetics, drugs, biologies and other chemicals.

Description

TITLE
CO-CULTURE LYMPHOID TISSUE EQUIVALENT (LTE) FOR AN ARTIFICIAL
IMMUNE SYSTEM (AIS) BACKGROUND OF THE INVENTION
Field of the Invention The present invention is directed to a method for constructing an integrated artificial human tissue construct system and, in particular, construction of an integrated human immune system for in vitro testing of vaccines, adjuvants, immunothentpy candidates, cosmetics, drugs, biologics, and other chemicals. The artificial immune system of the present invention is useful for assessing the interaction of substances with the immune system, and thus can be used to accelerate and improve the accuracy and predictability of, for example, vaccine, drug, biologic, immunotherapy, cosmetic, and chemical development.
Background of the Technology Despite the advent and promise of recent technologies, including combinatorial chemistry, high-throughput screening, genornics, and proteomics, the number of new drugs and vaccines reaching the market has not increased. In fact, the attrition rate within drug discovery programs exceeds 90%.
The introduction of these new (and expensive) technologies has not reduced the lost opportunity costs associated with immunotherapy development; rather, these costs have increased. Indeed, it is now estimated that almost $1 billion is required to bring a new drug to the market.
The development and biological testing of human vaccines has traditionally relied on small animal models (e.g., mouse and rabbit models) and then non-human primate models. However, such small animal models are expensive arid non-human primate models are both expensive and precious. Furthermore, there are many issues regarding the value of such animal studies in predicting outcomes in human studies.
A major problem remains the translation from test systems to human immunology. Successful transfer between traditional testing systems and human biology requires an intricate understanding of disease pathogenesis and immunological responses at all levels. Given worldwide health problems caused by known and emerging infectious agents and even potential biological warfare pathogens, it is time for a fresh approach to understanding disease pathogenesis, the development and rapid testing of vaccines, and insights gathered from such work.
The body's distributed immune system can be roughly divided into four distinct compartments: tissues and blood, mucosal tissues, body cavities, and skin.
Because of ease of study, most is known about the tissue and blood compartment and its lymphoid tissues, the spleen and lymph nodes.
The mammalian immune system uses two general adaptive mechanisms to protect the body against environmental pathogens. When a pathogen-derived molecule is encountered, the immune response becomes activated to ensure protection against that pathogenic organism.

2 The first immune system mechanism is the non-specific (or innate) inflammatory response. The innate immune system appears to recognize specific molecules that are present on pathogens but not within the body itself.
The second immune system mechanism is the specific or acquired (or adaptive) immune response. Innate responses are fundamentally the same for each injury or infection; in contrast, acquired responses are custom-tailored to the pathogen in question. The acquired immune system evolves a specific immunoglobulin (antibody) response to many different molecules, or antigens, derived from the pathogen.
In addition, a large repertoire of T cell receptors (TCR) is sampled for their ability to bind processed peptides from the antigens that are bound by major histocompatibility complex (MHC) class I and II proteins on the surface of antigen-presenting cells (APCs), such as dendritic cells (DCs). Acquired immunity is mediated by specialized immune cells called B and T lymphocytes (or simply B and T cells).

Acquired immunity has specific memory for specific antigens; repeated exposure to the same antigen increases the memory response, which increases the level of induced protection against that particular pathogen.
B cells produce and mediate their functions through the actions of antibodies.
B
cell-dependent immune responses are referred to as "humoral immunity" because antibodies are found in body fluids.
=
T cell-dependent immune responses are referred to as "cell-mediated immunity," because effector activities are mediated directly by the local actions of effector T cells. The local actions of effector T cells are amplified through synergistic interactions between T cells and secondary effector cells, such as activated macrophages. The result is that the pathogen is killed and prevented from causing diseases.
The functional element of a mammalian lymph node is the follicle, which develops a germinal center (GC) when stimulated by an antigen. The GC is an active area within a lymph node, where important interactions occur in the development of an

3 effective humoral immune response. Upon antigen stimulation, follicles are replicated and an active human lymph node may have dozens of active follicles, with functioning GCs. Interactions between B cells, T cells, and FDCs take place in GCs.
Various studies of GCs in vivo indicate that the many important events occur there, including itnmunoglobulin (Ig) class switching, rapid B cell proliferation (GC
dark zone), production of B memory cells, accumulation of select populations of antigen-specific T cells and B cells, hypermutation, selection of somatically mutated B
cells with high affinity receptors, apoptosis of low affinity B cells, affinity maturation, .
induction of secondary antibody responses, and regulation of serum immunoglobulin G
(IgG) with high affmity antibodies. Similarly, data from in vitro GC models indicate that FDCs are involved in stimulating B cell proliferation with mitogens and it can also be demonstrated with antigen (Ag), promoting production of antibodies including recall antibody responses, producing chemokines that attract B cells and certain populations of T cells, and blocking apoptosis of B cells.
Similar to pathogens, vaccines function by initiating an innate immune response at the vaccination site and activating antigen-specific T and B cells that can give rise to long term memory cells in secondary lymphoid tissues. The precise interactions of the vaccine with cells at the vaccination site and with T and B cells of the lymphoid tissues are important to the ultimate success of the vaccine.
Almost all vaccines to infectious organisms were and continue to be developed through the classical approach of generating an attenuated or inactivated pathogen as the vaccine itself. This approach, however, fails to take advantage of the recent explosion in our mechanistic understanding of immunity. Rather, it remains an empirical approach that consists of making variants of the pathogen and testing them for efficacy in non-human animal models.
Advances in the design, creation and testing of more sophisticated vaccines have been stalled for several reasons. First, only a small number of vaccines can be tested in humans, because, understandably, there is little societal tolerance for harmful

4 side effects in healthy people, especially children, exposed to experimental vaccines.
With the exception of cancer vaccine trials, this greatly limits the innovation that can be allowed in the real world of human clinical trials. Second, it remains challenging to predict which immunodominant epitopes are optimal for induction of effective CD4+
and CD8+ T cell responses and neutralizing B cell responses. Third, small animal testing, followed by primate trials, has been the mainstay of vaccine development; such approaches are limited by intrinsic differences between human and non-human species, and ethical and cost considerations that restrict the use of non-human primates.
Consequently, there has been a slow translation of basic knowledge to the clinic, but equally important, a slow advance in the understanding of human immunity in vivo.
The artificial immune system (AIS) of the present invention can be used to address this inability to test many novel vaccines in human trials by instead using human tissues and cells in vitro. The AIS enables rapid vaccine assessment in an in vitro model of human immunity. The AIS provides an additional model for testing vaccines in addition to the currently used animal models.
Attempts have been made in modulating the immune system. See, for example, U.S. Patent No. 6,835,550 Bl, U.S. Patent No. 5,008,116, WO 2004/101773 Al, Suematsu et aL, (Nat Biotechnol, 22, 1539-1545, (2004)) and U.S. Patent Application No. 2003/0109042.
Nevertheless, none of these publications describe or suggest an artificial (ex vivo) human cell-based, immune-responsive system comprising a vaccination site (VS) and a lymphoid tissue equivalent (LTE). The present invention comprises such a system and its use in assessing the interaction of substances with the immune system.
SUMMARY OF TIM INVENTION
The present invention is directed to artificial immune systems comprising cell cultures of B cells, T cells and antigen-primed denciritic cells.

The present invention is also directed to methods for detecting an immune response to an antigen using the cell cultures of the present invention.
The present invention is also directed to a method for testing an immune response to an antigen in vitro comprising:
- preparing a cell culture comprising dendritic cells primed with an antigen;
- adding syngeneic B cells;
- adding allogeneic T cells; and - analyzing the effect said antigen has on the B cell response in the cell culture.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1: Shows the detection of tetanus-specific antibody responses by ELISPOT and determination of the percentage of antigen-specific B cells using a 2D T and B
cell co-culture.
FIGURE 2: Depicts tetanus toxoid: B cell proliferation and comparison between PBMC
and 2D T and B cell co-culture.
FIGURE 3: Shows the flow cytometry data indicating B cell proliferation between PBMC
and 2D T and B cell co-culture for the same cell donor shown in Figure 2.
FIGURE 4: Depicts tetanus toxoid-specific ELISPOT comparing PBMC to 2D T and B
cell co-culture for the same cell donor shown in Figures 2 and 3.
FIGURE 5: Shows an in vitro system representative of the physiological state promotes stronger B cell proliferative and tetanus toxoid-specific antibody responses, using a 2D co-culture of T and B cells and TT-pulsed DCs.
FIGURE 6: Depicts tetanus-specific antibody responses to a DTaP (diphtheria and tetanus and acellular pertussis vaccine, adsorbed) vaccine and a simple tetanus toxoid Antigen, using a 2D co-culture of T and B cells and TT-pulsed DCs.
FIGURE 7: Shows the influence of vaccine versus antigen in a lymphoid tissue equivalent (LTE) for the same cell donor shown in Figure 6.

FIGURE 8: Depicts Strong B cell and T cell proliferative responses seen against C.
albicans, associated with potent activation (HLA-DR'', CD86high) of the dividing B
cells using a 2D co-culture of T and B cells and TT-pulsed DCs.
. .
FIGURE 9: Shows specificity of the C. aibican-stimulated B cells demonstrated by ELIPSOT for the same donor in Figure 8. C albicans-specific ELISPOT data comparing compares the 2D co-culture of T and B cells with PBMCs.
FIGURE 10: Depicts antibody responses when some of the leukocytes are removed.
FIGURE 11: Shows in vitro antigen-specific antibody response to influenza.
FIGURE 12: Shows T and B cell proliferation induced by H1N1 influenza.
FIGURE 13: Shows in vitro the importance of the temporal sequence of immunological events.
FIGURE 14: Shows a rapid assay system for the development of primary B cell responses.
DETAILED DESCRIPTION OF THE INVENTION
The present invention concerns the development of accurate, predictive in vitro models to accelerate vaccine testing, allow collection of more informative data that will aid in redesigning and optimizing vaccine formulations before animal or clinical trials, and raise the probability that a vaccine candidate will be successful in human trials.
More specifically, the present invention comprises controlling the nature and state of the cells in the lymphoid tissue equivalent (LTE, artificial lymph node) of the artificial immune system (AIS).
=4. 7 The AIS can be used to test vaccines and other pharmaceuticals for immune reactivity in a manner that is more predictive than animal experiments.
Consequently, it can provide valuable pre-clinigal data earlier in the research and development process. Antigenic molecules introduced to the AIS are acquired by dendritic cells (DCs) at the vaccination site (VS). The DCs are then transferred to the lymphoid tissue equivalent (LTE), where they present the antigen to T cells, activating their immune function. Activated helper T cells co-stimulate B cells to induce antibody production, while activated cytotoxic T cells lyse antigen-bearing cells. Solubilized antigen(s) can also be introduced into the LTE to directly activate B cells for subsequent antibody production.
While a number of published reports have demonstrated antigen-specific B cell responses (to C. albicans, Ti', and other antigens) in vitro, these results are typically achieved by stimulating and restimulating cultures of whole PBMCs with antigen and exogenous factors to boost B cell proliferation and/or activation.
The present invention comprises the detection of antibody responses using defined cultures of B cells, T cells, and DCs and optionally follicular dendritic cells (FDCs), in 2-dimensional construct assay. The presence of secondary cells provides a more physiological environment for B cell activation and differentiation, such that artificial factors in the cultures are not necessary to detect specific antibody responses. In an embodiment of the present invention, the LTE comprises allogeneic T cells. In another embodiment, the LTE comprises autologous T cells.
Using embodiments of the present invention, we have generated antigen-specific B cell responses using a 2-dimensional (2D) cp-culture system comprising T
cells, B cells, and antigen-pulsed DCs. In the examples, responses were generated against tetanus toxoid (TT) and a whole protein extract of Candida albicans (C.
albicans). The results from these examples show that culturing human T and B
cells together in vitro at a ¨1:1 ratio, versus the ratio of T and B cells naturally found in the blood, gave stronger antigen responses, by both analysis of activation and proliferation (flow cytometry) and antibody production (ELISPOT). Although the preferred ratio of T cells:B cells is ¨1:1, the ratio of T cells:B cells can range from ¨1:10 to ¨10:1. In the cultures of the examples, "T cells" included both CD4+ and CD8+ T cells. In peripheral blood, the T (total T cells):B cell ratio is ¨7:1. In the lymph node, the T
(total T
cells):B cell ratio is ¨1:1.6. In the germinal center, the T cell:B cell ratio is ¨1:8, and there, the T cells are primarily CD4+ T cells.
In the results of the experiments shown, engineered serum-free media (X-VIVO) was used, though we have also used serum (e.g., human, bovine) in other experiments (data not shown). Dendritic cells (DCs) were generated from CD14-purified monocytes that were cultured for ¨7 days in X-VIVO 15 media, supplemented with GM-CSF (-100 ng/ml) and IL-4 (-25 ng/ml). The cytokine-derived DCs were pulsed with antigen or vaccine and then cocultured with T cells. After adding the antigen-prepulsed dendritic cells to the T cell culture, B cells primed with the same antigen used to prime the DCs are added to the dell culture. After adding the antigen-primed B cells to the cell culture, further soluble antigen can also be added.
For PBMC cultures, either the antigen was added to the assay, or antigen-pulsed DCs were added to the assay. In Figures 1 to 9, antigen-pulsed DCs were added to the co-culture of T and B cells, while soluble antigen was added to the PBMC cultures. Figure shows a comparison of the co-culture to PBMCs, with antigen-pulsed DCs added to both systems.
Antibodies specific for the antigen of interest can be isolated from the resulting cell cultures of the present invention. Such antibodies can be used for a variety of purposes, including in therapeutic and diagnostic methods.
In addition, antigen-specific B cells can be isolated, cloned and immortalized from the cell cultures of the present invention, and can also be used in therapeutic and diagnostic methods.

Alternatively, all of the antibody-producing B cells are collected en masse (without isolating/cloning individual B cells) for immortalizing and producing a therapeutic.
Thus, the present invention also encompasses methods of producing a therapeutic comprising antibodies or antibody-producing B cells in combination with a pharmaceutically acceptable carrier.
Examples These experiments provide a direct comparison of PBMCs versus a co-culture of negatively selected T and B cells that were plated at a ¨1:1 ratio in ¨ in these examples ¨ a 96-well, round bottom plate. All assays were harvested on day 7 of in vitro culture. All experiments were analyzed by ELISPOT for antibody production and by flow cytometry for proliferation, as determined by loss of CFSE. In the ELISPOT
assays because there were different ratios of T and B cells in the PBMC
culture compared with the TB-2D cultures, there were fewer B cells plated into the ELISPOT
wells. However, in the experiment in Figure 4, the numbers of B cells used in the ELISPOT experiments for both the PBMC and co-culture assays were approximately equal. We determined the approximate number of B cells in the ELISPOT wells by flow cytometry to enable comparisons.
These results show that culturing human T and B cells together in vitro at a ¨1:1 ratio compared to the ratio of T and B cells naturally found in the blood give stronger antigen responses, by both analysis of activation and proliferation (flow cytometry) and antibody production (ELISPOT). The results also show that co-culturing T cells and antigen-primed dendritic cells, and subsequently adding antigen-primed B
cells, also gives stronger antigen responses, as indicated by antibody production (ELISPOT).

Example 1.
B and T cell co-culture with tetanus toxoid, showing the ability to detect tetanus-specific antibody responses.
Example 2a.
PBMC versus co-culture, using a tetanus toxoid antigen. Even though similar B
cell proliferation responses were seen in PBMC and 2D T and B cell co-cultures (Figures 2, 3), an improved tetanus toxoid-specific antibody response was observed in a T and B cell co-culture LTE, as compared with PBMC cultures (Figure 4).
Example 2b.
PBMC versus co-culture, using Candida albicans antigens. Figure 9 shows C.
albicans-specific ELISPOT data, comparing TB-2D to PBMCs. In this experiment, DCs were pulsed with Ti' antigen only, but the ELISPOT was conducted on both TT-and C. albicans-coated plates.
Example 2c.
PBMC versus co-culture (Figure 10). In this example we addressed the question of what happens if we take cells from an apparent "non-responder" and use only the GC cells from the leukocytes. Note the response when some of the leukocytes are removed (Figure 10); non-responders in vitro now show an antibody response.
Here, we used human CD4+ T and B cells with FDCs and formed GCs in vitro and then examined whether IgG production could be obtained against a recall antigen.
Specifically, we used tetanus toxoid (Ti') in these experiments and isolated human B
cells and CD4+ T cells from peripheral blood.
We observed IgG recall responses using only the T cells, B cells, and FDCs that are typically found in GCs. In contrast, in the presence of PBL cells not normally in found in GCs, no antibody response was detectible in cells from some donors.
These =

results show that removing (not including) other cells, such NK cells, monocytes, and CD8+ T cells, improved the IgG response.
Example 3 In vitro system representative of the physiological state promotes higher B
cell proliferative and tetanus toxoid-specific antibody responses following tetanus vaccination (Figure 5). The post tetanus toxoid experiment was conducted 5 weeks following vaccination. The tetanus antibody titer before vaccination was ¨40 g/triL;
after vaccination it was ¨30Oug/mL. T cells represent both CD4+ and CD8+ T
cells.
Peripheral blood has a T:B ratio of ¨7:1 (total T cells). The lymph node has a T:B ratio of ¨1:1.6 (total T cells). The germinal center has a T:B ratio of ¨1:8 (primarily CD4+ T
cells).
Example 4 Use of a vaccine to elicit in vitro immune responses in a co-culture of T and B
cells. DCs were pulsed with the vaccine ot the tetanus toxoid antigen and were then added to the co-culture of T and B cells. Tripedia (diphtheria and tetanus toxoids and acellular pertussis vaccine, adsorbed; DTaP), for intramuscular use, is a sterile preparation of diphtheria and tetanus toxoids adsorbed, with acellular pertussis vaccine in an isotonic sodium chloride solution containing thimerosal (preservative) and sodium phosphate (to control pH). After shaking, the vaccine is a homogeneous white suspension. Tripedia vaccine is distributed by Aventis Pasteur Inc.
Example 5 To detect antigen-specific antibody responses, we developed an ELISPOT
approach to quantify B cell responses (antigen specificity) on a per cell basis. In this example, T cells were cultured with B cells at a ¨1:1 ratio, with cytokine-derived DCs included at a DC:T and B (total) cell ratio of ¨1:60. Soluble TT (-1 itg/m1) or C.
= 12 albicans (-10 gginal) was included for the entire 7-day culture, while other wells received pokeweed mitogen (PWM; a strong, non-specific lymphocyte stimulator) for the final 3 days of the culture.
On the seventh day, the lymphocytes were examined for marker expression and CFSE profiles by flow cytometry and the frequency of TT and C. alb/can-specific B
cells was calculated by ELISPOT. Briefly, ¨30x103 total lymphocytes were plated in duplicate wells of an ELISPOT plate that had been pre-coated with 'IT, C.
albicans, or anti-immunoglobulin (Ig, to gauge total antibody production).
The cells were then serially diluted five times at a ¨1:3 ratio and PWM was added to all wells to trigger antibody production. The cells were then incubated for ¨5 hr at 37 C in a 5% CO2 incubator and washed away. Plate-bound antibody was detected using techniques .similar to those required for ELISA.
The results in Figure 8 demonstrate strong B cell and T cell proliferative responses against C. albicans, associated with potent activation (HLA-DR", CD86high) of the dividing B cells. Furthermore, a subset of the most divided B
cells appears to have acquired a memory phenotype, indicated by increased CD27 expression.
The lack of a robust response against IT was consistent with the weak serum IT titer for this donor (-4ug/m1). As expected, PWM triggered potent T and B
cell proliferative responses, though not as many divisions were seen as with specific antigen stimulation, likely because the cells were only cultured with the mitogen for 3 days.
The specificity of the C. albicans-stimulated B cells was demonstrated by ELIPSOT (Figure 2). This experiment suggests that a lx stimulation with C.
albicans did give rise to a small population of antibody-producing cells (-0.2% of total B cells) that was not detected 'in untreated cultures or those stimulated with IT (left and middle wells). This discrepancy between the frequency of proliferating cells and C.
albicans-specific B cells detected by ELISPOT could be the result of several factors. A
likely explanation is that we used a crude C. albicans whole antigen extract containing ¨19%

carbohydrates (by weight). While C. albicans polysaccharides are strong inducers of B
cell responses, only protein antigen-specific responses would be detected in the ELISPOT assay.
Example 6 Tetanus-specific antibodies were detected in another ELISPOT experiment where the cell donor's serum anti-tetanus level was higher (63 g/m1), and DCs were cultivated in XVIVO-15 medium. All other components, concentrations and ratios were left unchanged, except that of the number of cells deposited per ELISPOT
well was increased; the higher number used was --1x105 cells/well.
In this experiment, both IT- and C. albicans-specific antibodies were observed (up to 48 and 33 spots per well, respectively), although a high level of non-specific response, especially in the presence of CCL21/anti-CD40 additives, did not allow a firm conclusion in favor of antigen-specific versus mitogenic activity.
Example 7 The specificity of the C. albicans-stimulated B cells was demonstrated by ELIPSOT (Figure 9) for both PBMC and 2D co-culture of T and B cells with C.
aThicans-pulsed DCs added to both systems. This experiment indicates that even if the PBMC cultures have antigen-pulsed DCs added that the co-culture system shows a stronger antibody response, as determined by ELISPOT.
Example 8 In vitro antigen-specific antibody response to influenza (Figure 11) and T and B
cell proliferation induced by H1N1 influenza (Figure 12). DCs were treated (or not) with H1N1 (New Caledonia) influenza. 20 cultures of DCs and T and B cells were stimulated (or not) with 'soluble' H1N1 influenza. As can be seen, there was antigen-specific proliferation of T and B lymphocytes and generation of antigen-specific antibody secreting B lymphocytes (ELISPOT data). Note the largest (apparently synergistic) response was observed when we pulsed the DCs with antigen and then added soluble antigen to the DC / T and B cell cultures, to activate the B
cells, which are antigen-presenting cells (APCs). Again, the T and B cell co-culture is superior to PBMC cultures.
Example 9 In this example, the antigens examined were tetanus toxoid (TI) and a whole protein extract of Candida albicans.
CD4+ T cells, purified by negative magnetic bead selection, were cultured with antigen-pulsed DCs at ¨1:60 ratio.
In the first experiment, B cells were added at the same time (day 0) in equal numbers to the T cells, DCs and antigen.
In the second experiment, antigen-primed B cells were added to the T/DC
cultures 3 days after the T/DC cultures were established.
As a control, cultures were also established in the absence of any antigen.
Seven days following the addition of the antigen-primed B cells to the T/DC
cultures, the cells were harvested and analyzed for antigen-specific B cells by ELISPOT assay.
As Figure 13 illustrates, the immune (antibody) response was much stronger in the case where the antigen-primed B cells were added to the T/DC culture after 3 days.
Similar results were obtained using cells obtained form three independent blood donors.
Example 10 A rapid assay system for the development of primary B cell responses.
Antigen-primed dendritic cells (DCs) were cultured with syngeneic B cells (-2 to ¨2.5x106) in 24-well plates and allogeneic CD4+ T cells at a 1:100 ratio (i.e., ¨2 to ¨2.5x104 allogeneic T cells). Additional antigen was added (the antigens used were keyhole limpet hemocyanin (KLH) and the anthrax recombinant protective antigen (rPa)). Fourteen days later, the primary B cell response was assessed by ELISPOT and FACS. As Figure 14 illustrates, the ELISPOT results show that, despite some background, enhanced primary B cell responses (IgM) were observed to the antigens. =
While the foregoing specification teaches the principles of the present invention, with examples provided for the purpose of illustration, it will be appreciated by one skilled in the art from reading this disclosure that various changes in form and detail can be made without departing from the true scope of the invention.

Claims (6)

WHAT IS CLAIMED IS:

1. A method for testing an immune response to an antigen in vitro comprising:
- preparing a cell culture comprising dendritic cells primed with an antigen;
- adding syngeneic B cells;
- adding allogeneic T cells; and - analyzing the effect said antigen has on the B cell response in the cell culture.

2. The method of claim 1, further comprising adding additional antigen together with the allogeneic T cells to the cell culture.

3. The method of claim 1, wherein the T cells and B cells are present in the culture in a ratio of 1:100, respectively.

4. The method of claim 1, wherein the cell culture was cultured for 14 days prior to the analyzing.

5. The method of claim 1, wherein the analyzing is by ELISPOT and FACS.

6. The method of claim 1, wherein the B cell response is production of IgM.